Air preheater for steam generating plants



Dec. 26, 1950 D. DALIN ET AL AIR PREHEATER FOR STEAM GENERATING PLANTSFiled Nov. 22, 1944 Patented Dec. 26, 1950 AIR PREHEATER FOR STEAMGENERATING PLANTS David Dalin', Ronninge, and Gustav Vilhelm. Ha-gby,Ostertalje, Sweden Application November 22, 1944, Serial No. 564,584 InSweden November 6, 1943 1 Gl'aim. 1

The present invention relates to a heat exchange apparatus and refersmore particularly to a heat exchanger especially adapted for airpreheating by means of hot flue gases.

The object of this invention is to provide an efficient. simple andcompact air preheater by which desired overall plant efficiency can bematerially improved without producing undesirable gas pressure drop ordraft losses within the flue gas passage.

While past preheater constructions have employed tubes as a means forexchanging heat be tween the hot gases and the air to be heated, it hasbeen consistently the practice to have both ends of the tubes anchoredto tube sheets. As a result, the stresses resulting from expansion andcontraction of the tubes constantly loosen the tube sheets.

The present invention overcomes this object on by using U-shaped tubesanchored only at their open ends with the turn or bight oi the Uentirely free;

A further object of the present invention is to provide an air preheaterunit which may be very easily installed or removed from its positionwithin a flue gas passage.

With the above and other objects in view, which will appear as thedescription proceeds, this invention resides in the novel constructionand arrangement of parts substantially as hereinafter described, andmore particularly defined by the appended claims, it being understoodthat such changes in. the. precise embodiment of the here nafterdisclosed invention may be made as come within the scope of the claims.

The accompanying drawing illustrates one complete example of thephysical embodiment of the invention constructed in accordance with thebest mode so far devised for the. practical application of theprinciples thereof, and: in which:

F gure 1 is a vertical sectional view of an air preheater embodying thepresent invention;

Figure 2 is an end view of the preheater apparatus taken from the outletend thereof; and

Figure 3 is a top plan view of the preheater apl paratus showing thearrangement of the biiurcated inlet and outlet air passages.

Referring now more particularly to the accompanying drawing in whichlike numerals indicate like parts the numeral l3 designates a section ofa duct hav ng an inlet 8 and an outlet 9 at opposite ends thereof. Theduct section serves as a passage through which hot flue gases pass andmay be located at any point along the flue gas passage, but preferablyat a point where the flue gas is the hottest, since the hotter the fluegas passing through the air preheater the less chance there is for theformation of condensation within the preheater.

Located within this section of the duct are three groups of U-shapedtubes 12, b and c, all of the tubes of which hang from a flat tube sheet4 which forms the topwall for the preheater portion of the flue gasduct. These groups of tubes are spaced from one another along the pathof the gases with group 11 located farthest downstream near the outlet9.

Above the top wall of the tube sheet a is a housing i I having aplurality of spaced apart straight vertical partition walls [2 whichextend transversely of the duct l3. These partition walls divide thehousing into six adjacent transverse headers, two of which aredesignated by the numeral l, two by the numeral 6 and the two remainingones i and 6. The headers land 6 are arranged alternately, the headers ibeing connected with a common air inlet chamber 21, located at one sideof the duct l3, and the headers 6 being connected with a common outletchamber 22, located at the other side of the duct l3.

Each of the tube groups is composed of a plurality of U-shaped tubes l,with the extremities of their legs 2 and 3 fixed to the tubes sheet 4 sothat the anchored upper ends of the tubes open to the headers. The tubeshang from the tube sheet A and have their closed ends or bights' 5located adjacent to the bottom Wall of the duct in the manner shown inFigure 1.

One leg of each of the U-shaped tubes of groups 1a, and b communicateswith an i-nletheader I and the other leg thereof communicates with theadjacent outlet header 6, upstream therefrom.

Thus, with each of the tubes of the groups a and I) having one endcommunicated with an inlet header 5 and its other end communicating withthe adjacent outlet header ii (upstream therefrom) it will be readilyunderstood that cold air entering the inlet 2! flows into both headersl, which may be considered branches of the inlet, through the U-shapedtubes of groups a and b to be heated by the hot flue gases flowingthrough the flue gas duct and over the tubes, then flows into the warmair headers 5 to be discharged through outlet 22. Attention is directedto the fact that the tubes are so disposed that their bights areparallel with the longitudinal axis of the flue gas duct, and that allof the tubes are wholly contained within the flue gas passage.

This construction permits the heating of large 3 quantities of airwithin a relatively short length of gas passage, and also enables thetubes to freely expand or contract without interfering with adjacenttubes and without affecting the tube sheet 4 to which the tubes aresecured.

In addition the tubes of a particular row of any one of the groups maybe replaced without necessitating removal of all of the tubes of asingle group.

Since the air to be heated enters at the inlet 2| and flows into thetubes from the headers 1 a desirable counterflow is achieved, i. e. theair as it is heated moves upstream with respect to the gas flow,especially as the air passes through the horizontal bights 5 of thetubes.

The third tube group c can be used in conjunction with the tube groups aand b or independently thereof. It is preferably located upstream of thetube groups a and b. Tube group 0 is similar in construction to groups aand b and has a cold air header 1 and a warm air header 6 with which thelegs of the U-shaped tubes of the group communicate. Circulation of airto be heated through tube group 0 is the same as through groups a and b,with cold air entering header 1 passing through the tubes and into theheader 6.

Normally the groups a and b are employed for heating primary air and thethird group 0 is employed for heating secondary air, since the secondaryair must necessarily be hotter than the primary air. The air preheatercomprising the present invention is primarily intended to be used inplants having stoker firing, for example in plants in which the primaryair need only be heated to a temperature between 102 C. and 105 C., butin which the temperature of the secondary air should be as high aspossible, and in which the quantity of primary air is considerablygreater than the quantity of secondary air.

In this type of plant and in the type of plants designed to usedifferent kinds of fuels, thus necessitating different quantities ofsecondary air, and in which the same grate is used in the combustion ofthe fuels, there may be provided a connecting duct with regulatingvalves between the secondary air outlet duct and the primary air outletduct. Such a connecting air duct is indicated by 23 in Figure 3 of thedrawing. The use of connecting passage is usually required when thequantity of secondary air needed for the boiler varies, and serves inthe type of apparatus illustrated to transfer a portion of the secondaryair to the primary air passage. In this way that portion of the airpreheater intended for heating of secondary air may always be completelyutilized even though the quantity of secondary air required by theboiler is comparatively small.

From the foregoing description taken in connection with the accompanyingdrawing, it will be apparent to those skilled in the art that thisinvention provides an air preheater having many advantages over devicesof this type heretofore available, since the air preheater unit is socompact as to require very limited space and to permit highly efiicientheat exchange between the flowing media.

Having now described our invention, what we claim as new and desire tosecure by Letters Patent is:

An air preheater for steam generating plants comprising: a duct throughwhich hot gases are adapted to flow, said duct having a flat top wall atone portion thereof; two groups of U-shaped tubes wholly inside the ductand one downstream from the other, said tubes having the extremities oftheir legs fixed to said flat top wall portion and opening therethrough,and each group of tubes extending across the duct from side to sidethereof with the legs of the tubes of each group lying at opposite sidesof a plane transverse to the duct so that the tubes occupy planesextending substantially in the direction of gas flow in the duct; ahousing connected to said top wall portion at the exterior of the duct;spaced apart straight walled partitions in said housing extendingtransversely of the duct and located between the groups of tubes andsubstantially in said transverse planes between the legs of the tubes ofeach group, said partitions defining two pairs of separate but adjacentheaders, each header extending transversely across said fiat top wallportion of the duct over and in communication with the open ends of thetubes at one side of said transverse plane; an air inlet chambercommunicating with one set of alternate headers at one side of the duct;and an air outlet chamber communicating with the other set of alternateheaders at the opposite side of the duct.

DAVID DALIN GUSTAV VIIJ-LELM HAGBY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,421,247 Keenan, Jr June 27,1922 1,673,122 Mills June 12, 1928 1,744,452 Dowd, Jr Jan. 21, 19301,765,675 Jacobus June 24, 1930 1,833,314 Bruce Nov. 24, 1931 1,919,029Lucke July 18, 1933 2,008,255 Larkin July 16, 1935 2,029,284 ArmacostFeb. 4, 1936 2,063,441 Kerr Dec. 8, 1936 2,279,518 Olson Apr. 14, 19422,423,997 Ruegg July 15, 1947

